In wellbores, downhole annular barriers are used for different purposes, such as for providing a barrier for flow between an inner and an outer tubular structure or between an inner tubular structure and the inner wall of the borehole. The downhole annular barriers are mounted as part of the well tubular structure. A downhole annular barrier has an inner wall surrounded by an annular expandable sleeve. The expandable sleeve is typically made of an elastomeric material, but may also be made of metal. The sleeve is fastened at its ends to the inner wall of the downhole annular barrier.
In order to seal off a zone between an inner and an outer tubular structure or a well tubular structure and the borehole, a second annular barrier is used. The first annular barrier is expanded on one side of the zone to be sealed off, and the second annular barrier is expanded on the other side of that zone, and in this way, the zone is sealed off.
The quality of the seal of a sealed off zone is often defined by the flow of borehole fluids passing a seal, e.g. the requirements of a certain seal may be a maximum limit of a few litres per minute passing the seal to meet the requirements set up by the user. Therefore, a certain level of fluid leaking into or away from the sealed of zone is typically allowed and acceptable, but the quality of the seal is compromised if too much fluid can pass the seal.
When annular barriers are expanded, they typically tend to spring back when the expansion has ended. The spring back effect occurs when the pressure on the expandable part used to expand the expandable part is terminated. Termination of the expansion pressure will result in a small decrease in size of the expandable part due to elastic retraction of the expanded material. Also, other settling effects, such as pressure equalisation in the annular barrier, may cause a minor minimisation of the size of the barrier. Even when using metals, such as steel, a spring back effect of a few percent may be expected. The spring back effect of the expandable part negatively affects the quality of the seal provided by the downhole annular barrier 1, since the seal becomes poorer after expansion in terms of tightness or the amount of fluid possibly passing the seal.
It is thus desirable to provide a solution whereby the problems caused by spring back effects and other settling effects of the annular barrier material after expansion can be avoided.